Novel corrosion experiments using the wire beam electrode. (IV) Studying localised anodic dissolution of aluminium

An electrochemically integrated multi-electrode array namely the wire beam electrode (WBE) has been applied in novel experiments to study the anodic dissolution behaviour of aluminium (AA1100), which was exposed to corrosive media with and without the presence of corrosion inhibitor potassium dichromate. The objective of this work is to demonstrate the applicability of the WBE for investigating corrosion processes under anodic polarisation. Anodic current measurements and mapping have been made, for the first time, with the WBE surface being anodically polarised. Pitting potential as well as anodic dissolution profile has been successfully determined by mapping anodic dissolution currents over the anodically polarised WBE surface. The pitting potential determined using the WBE method was found to correlate well with that determined using the conventional pitting scan method; and the anodic dissolution profile determined using the WBE method was found to correlate with maps obtained using the scanning reference electrode technique (SRET). Potassium dichromate was found to significantly affect the pitting potential, anodic dissolution profile and pitting initiation characteristics of aluminium. Two mechanisms of localised corrosion initiation have been identified. For WBE surface under free corrosion or low anodic polarisation conditions, the initiation of localised corrosion was found to be due to the disappearance of minor anodes, which lead to accelerated dissolution of a few major anodes. For WBE surface under large anodic polarisation, the initiation of localised corrosion was found to be due to the formation of active new anodic sites. This work suggests that the WBE method is useful for understanding the electrochemical behaviour of localised anodic processes, and their dependence on externally controllable variables.     

Novel corrosion experiments using the wire beam electrode: (II) Monitoring the effects of ions transportation on electrochemical corrosion processes

An electrochemically integrated multi-electrode system namely the wire beam electrode (WBE) has been applied for the first time to study the effects of the transportation of electrochemically active species on the process, rate and pattern of electrochemical corrosion. The objective of this work is to demonstrate the applicability of the WBE method for investigating ion transportation related corrosion processes. A series of experiments have been carried out using WBEs made from mild steel and stainless steel wires. The WBE working surfaces were exposed to simulated diffusion-controlled corrosion environments where there were diffusion induced ions concentration gradients (termed diffusion-corrosion environment). Corrosion potential and current distribution maps (CPCD maps) were measured from WBE surfaces in continuous bases. Typical patterns have been identified from CPCD maps and the characteristics of these patterns have been found to depend heavily upon the type of electrode material and the type of corrosive ion. For mild steel WBE surface exposed to a diffusion-corrosion environment containing NiSO₄ or FeCl₃, the characteristic pattern in CPCD maps was found to emulate NiSO₄ or FeCl₃ concentration gradients, suggesting an ion-concentration controlled corrosion behaviour. However, when the mild steel WBE surface was exposed to a diffusion-corrosion environment containing NaCl, the characteristic pattern was found to show higher cathodic currents along the WBE edges with the magnitude decreasing in a contour-like manner towards the centre of the WBE surface, suggesting an oxygen concentration-controlled corrosion behaviour. When a stainless steel (SS316L) WBE surface was exposed to a diffusion-corrosion environment containing NiSO₄ or NaCl, the corrosion pattern appeared to be mainly determined by the random distribution of weak sites in passive film. When the SS316L WBE was exposed to a diffusion-corrosion environment containing FeCl₃, the CPCD map revealed a characteristic pattern that shows localised damage to passive film. This work demonstrates that the recognition and analysis of characteristic maps from WBE measurements can be used as a means of studying diffusion, migration and other forms of mass transportation related electrochemical corrosion processes.     

Novel corrosion experiments using the wire beam electrode: (III) Measuring electrochemical corrosion parameters from both the metallic and electrolytic phases

The wire beam electrode (WBE) and the scanning reference electrode technique (SRET) have been applied in a novel combination to measure, for the first time, electrochemical parameters simultaneously from both the metallic and electrolytic phases of a corroding metal surface. The objective of this work is to demonstrate the application of this combined WBE-SRET method in obtaining unique information on localised corrosion mechanism, by investigating typical corrosion processes occurring over a mild steel WBE surface exposed to the classic Evans solution. The WBE method was used to map current and potential distributions in the metallic phase, and the SRET was used to map current or potential distribution in the electrolytic phase. It has been found that the combined WBE-SRET method is able to gain useful information on macro-cell electrochemical corrosion processes that involve macro-scale separation of anodes and cathodes. In such macro-cell corrosion systems, maps measured using WBE and SRET were found to correlate with each other and both methods were able to detect the locations of anodic sites. However the movement of the scanning probe during SRET measurements was found to affect the SRET detection of cathodic sites. In micro-cell corrosion systems where the separation of anodic and cathodic sites were less distinct, SRET measurement was found to be insensitive in detecting anodic and cathodic sites, while the WBE method was still able to produce results that correlated well with observed corrosion behaviour. Results obtained from this work suggest that the WBE-SRET method is applicable for understanding the initiation, propagation and electrochemical behaviour of localised corrosion anodes and cathodes, and also their dependence on externally controllable variables, such as solution pH changes and the existence of surface coatings.     

An experimental comparison of three wire beam electrode based methods for determining corrosion rates and patterns

Laboratory experiments have been carried out to examine the advantages and limitations of three wire beam electrode (WBE) based techniques, including the noise resistance R_n-WBE method, the overpotential-galvanic current method, and the galvanic current method, in determining corrosion rates and patterns. These techniques have been applied simultaneously to several selected corrosion systems of different characteristics. It has been found that the R_n-WBE method has advantages over other WBE based methods when applying to WBE surfaces under uniform corrosion. However, the R_n-WBE method has been found to be unsuitable for low noise level corrosion systems. It has also been found that both R_n-WBE and overpotential-galvanic current methods are similarly applicable to WBE surfaces under nonuniform corrosion. However, the galvanic current method has been found to be suitable only for WBE surfaces under highly localised corrosion. Some related issues regarding R_n calculation such as trend removal and its effects on corrosion mapping have also been discussed.     

A new method of studying buried steel corrosion and its inhibition using the wire beam electrode

An electrochemically integrated multi-electrode system namely the wire beam electrode (WBE) has been applied for the first time to study corrosion of mild steel buried in sand, with and without the presence of corrosion inhibitor potassium dichromate (K₂Cr₂O₇). Measurements of galvanic current distribution maps have been carried out during the exposure of the WBE to dry, damp and chlorinated sand environments. Characteristic changes in galvanic current distribution maps have been observed during the initiation and propagation of localised corrosion. Specifically, during corrosion initiation in damped sand, new anodes were found to initiate and corrosion appeared to be in general form. When the WBE was later exposed to chlorinated sand, massive disappearance of anodic sites was found to occur, resulting in accelerated anodic dissolution of a small number of remaining anodic sites. Addition of corrosion inhibitor K₂Cr₂O₇ to the sand environment was found to significantly reduce galvanic current only after an initial increase in galvanic current. This result suggests that K₂Cr₂O₇ behaved as an anodic inhibitor.     

Correlation of temperature with galvanic corrosion behaviour of copper alloys based on wire beam electrode

Galvanic corrosion behaviour of copper and copper alloys in chloride solution (0.6?M NaCl) under different temperature conditions (35°C, 55°C and 80°C) was investigated by the wire beam electrode (WBE) method. It was found that the average galvanic current of the Cu and Cu–Sn alloy, which shifted from the cathode at 35°C to the anode at 80°C, increased as temperature increased during the immersion time. In contrast, the average galvanic current of Cu–Zn alloy can be ranked as 35°C?>?55°C?>?80°C. A sudden conversion was that the anode average current of Cu–Zn alloy at 35, 55°C changed into the cathode when the immersion time lasted untill 3 h at 80°C. The results indicate that it is feasible to study the corrosion behaviour under the different temperature environment with the WBE method.     

Emulation and study of the corrosion behavior of Al alloy 2024-T3 using a wire beam electrode (WBE) in conjunction with scanning vibrating electrode technique (SVET)

When a metal substrate surface is metallurgically non-uniform, heterogeneous electrochemical processes occur. The wire beam electrode (WBE) is a multi-piece electrode constructed with a variable number of metal wires embedded in insulating material. Each wire surface is much smaller than the total electrode surface, and for most purposes its corrosion and other electrochemical parameters can be assumed to be uniform even if the process on the whole electrode surface is not. With this assumption, all the electrochemical theories based on a uniform corroding surface and on uniform electrochemical processes can be applied to individual wires on the surface of a WBE. Two different types of WBE’s have been built with the intention of emulating the behavior of a heterogeneous aluminum alloy surface and have been used to extract the electrochemical noise resistance (R_n). A new approach is the use of a 9-wire WBE where the current density at the surface is detected by the scanning vibrating electrode technique (SVET). The ability of SVET to obtain data with small spatial resolution is used to understand if the WBE configuration can simulate the behavior of a plate electrode. From the data obtained thus far, the WBE configuration showed the ability to emulate a continuous surface as the current distributions from the analysis with the SVET has been confirmed.     

Assessment of intergranular corrosion (IGC) in 316(N) stainless steel using electrochemical noise (EN) technique

In the present work, an attempt was made to demonstrate the use of electrochemical noise (EN) technique in assessment of intergranular corrosion (IGC) in 316(N) stainless steel (SS). Degree of sensitization (DOS) in the specimens aged at 923 K for 24, 40, 50 and 100 h was determined using double loop electrochemical potentiodynamic reactivation (DLEPR) technique. Immediately after applying a prior preconditioning treatment, current and potential noise measurements were made. The DOS was determined from standard deviation of current noise (σ_I) versus time plot and it was found to bear a good correlation with the values obtained by DLEPR. Shot noise analysis of the EN data confirmed the above results.     

Study of anti-contamination performance of temporarily protective oil coatings using wire beam electrode

A novel electrochemical method was developed to investigate the anti-contamination performance of temporarily protective oil coatings. Results showed that salt contamination on the metal substrate had influence on the heterogeneous distributions of corrosion potential, polarization resistance and their regularities. With increasing of salt contamination degree, the corrosion potentials distribution of oil coatings followed a discontinuous binomial probability distribution, whereas the anodic polarization resistance (R_a) distribution of oil coatings transformed from a log-normal probability distribution to an exponential probability distribution and then to a discontinuous binomial probability distribution, the cathodic polarization resistance (R_c) distribution of oil coatings followed a log-normal probability distribution. The influence of salt contamination on R_a and R_c of oil coatings was different. It was suggested that it is helpful to study the process of underfilm corrosion, and evaluate the performance of contamination resistance of oil coatings by using wire beam electrode.     

Studying the formation process of chromate conversion coatings on aluminium using continuous electrochemical noise resistance measurements

A unique electrochemical technique, namely continuous noise resistance calculation (CNRC), was used to obtain electrochemical kinetic information from the formation process of chromate conversion coatings (CCC) on aluminium electrodes. It was found that the noise resistance (R_n) of aluminium electrodes remained almost unchanged during electrodes' immersion in a chromate containing acidic solution where the CCC films were supposed to form rapidly. This result indicates that the formation of CCC was associated with continuous corrosion of the aluminium electrodes and that the CCC films formed on aluminium surface were not intact barrier films, but most likely porous layers. The CCCs became protective only after they were aged in the environment. Based on these findings, the formation and inhibition mechanisms of CCC have been discussed.     

Detection of corrosion degradation using electrochemical noise (EN): review of signal processing methods for identifying corrosion forms

Electrochemical noise (EN), as one of the most promising in situ electrochemical methods in corrosion and electrochemical science, has been developing rapidly in recent years with the advancements in instrumentation and signal processing methods. One advantage of EN is its application in long-term or early stage corrosion process monitoring because it instantly detects corrosion rate and corrosion forms. Investigators have applied various mathematical methods to extract characteristic parameters from EN. In this paper, identifying corrosion forms using parameters obtained from time domain, frequency domain and time–frequency domain is reviewed, and the correlation between parameters and corrosion forms is discussed. Finally, other in situ techniques are recommended to be employed synchronously with EN measurement in order to obtain reliable analyses.     

Study of corrosion behaviour of mild steel and copper in thin film salt solution using the wire beam electrode

A novel electrochemical method was developed to study of corrosion behaviour of mild steel and copper in thin salt solution film. In this paper, mild steel and copper wire beam electrodes were used to allow corrosion potential in the thin salt solution film to be measured directly. It was found that the distribution of corrosion potentials in the thin salt solution film is heterogeneous compared with a naked wire beam electrode in the bulk salt solution. At the early stage of the corrosion, the distribution of corrosion potentials in the thin salt solution film gradually shifted to a higher potential range. With increasing of immersion time, the distribution of corrosion potentials reached to a highest potential range, then it began to shift negatively to a lower potential range. It was pointed out that the distribution of corrosion potentials of copper in the thin salt solution film could be improved with increasing the degree of corrosion. However, for the steel wire beam electrode, with increasing the degree of corrosion, the distribution of corrosion potentials in the thin salt solution film tended towards heterogeneous distribution.     

Understanding electrodeposition of polyaniline coatings for corrosion prevention applications using the wire beam electrode method

Polyaniline (PANI) films have been successfully electrodeposited on aluminium AA1100 electrode surfaces in acidic electrolytes as anti-corrosion coatings. The wire beam electrode (WBE) has been applied for the first time as a novel tool to monitor the electrodeposition processes; and also to understand the anti-corrosion performance and mechanism of the PANI coatings. During PANI electrodeposition, the WBE was polarised anodically, and anodic polarisation currents were measured from various locations over the WBE surface to produce anodic polarisation current maps (APC maps). Preliminary experiments have revealed that if an AA1100 electrode was not pre-treated, APC maps would show a localised anodic current distribution, resulting in a nonuniform PANI deposit. If the AA1100 electrode was pre-treated by a cathodic polarisation process, APC maps would show a random anodic current distribution, leading to a PANI coating covering the whole electrode surface. When exposed to a corrosive environment, PANI coatings were found to prevent localised corrosion of AA1100, primarily by enhancing its passive film rather than by a barrier mechanism. However, a nonuniform PANI coating has been found to accelerate general corrosion of AA1100. These results suggest that the WBE is a practical tool for monitoring, characterising and optimising PANI electrodeposition processes and for evaluating the anti-corrosion performance of PANI coatings.     

The influence of the cathodic process on the interpretation of electrochemical noise signals arising from pitting corrosion of stainless steels

The use of electrochemical noise (EN) measurements for the investigation and monitoring of corrosion has allowed many interesting advances in the corrosion science in recent years. A special advantage of EN measurements includes the possibility to detect and study the early stages of localized corrosion. Nevertheless, the understanding of the electrochemical information included in the EN signal is actually very limited. The role of the cathodic process on the EN signals remains uncertain and has not been sufficiently investigated to date. Thus, an accurate understanding of the influence of the cathodic process on the EN signal is still lacking. On the basis of different kinetics of the oxygen reduction it was established that the anodic amplitude of transients arising from pitting corrosion on stainless steel can be decreased by the corresponding electron consumption of the cathodic process. Therefore, the stronger the electron consumption, the weaker the anodic amplitude of the EN signal becomes. EN signals arising from pitting corrosion on stainless steel can be measured because the cathodic process is inhibited by the passive layer. This was confirmed by means of EN measurements under cathodic polarisation. Since the cathodic process plays a decisive role on the form of transients arising from pitting corrosion, its influence must be considered in the evaluation and interpretation of the EN signals.     

Study of localized corrosion of 304 stainless steel under chloride solution droplets using the wire beam electrode

Localized corrosion of 304 stainless steel under droplets of 1 M sodium chloride solution was investigated by the wire beam electrode (WBE) method. It was found that the current distributions were heterogeneous with isolated anodic current peaks mostly located near the edge of the droplet. During the corrosion process, the stainless steel WBE exhibited the stochastic characteristics with the disappearance of some anodic sites. In addition, stainless steel suffered more serious localized corrosion with the increase of the droplet size. The increase of the cathodic area and the three-phase boundary (TPB) length was believed to be the reason.     

The application of electrochemical noise resistance to evaluate the corrosion resistance of AISI type 304 SS in nitric acid

The paper presents the application of noise resistance to evaluate the corrosion behaviour of sensitized AISI type 304 SS in nitric acid of varying concentration (4 N, 12 N, 16 N) and temperature (298 K, 323 K, 348 K). Electrochemical noise data was acquired from a three identical electrode configuration in the required conditions at open circuit potential. The noise resistance was evaluated as the ratio of the standard deviation of the potential to that of the current noise after removing the DC component. The inverse relationship between noise resistance and corrosion rate was exploited to qualitatively assess the corrosion behaviour of AISI type 304 SS in nitric acid. Noise resistance decreased with increase in concentration implying an increase in corrosion rate with increase in nitric acid concentration. An increase in temperature from 298 K to 323 K and 348 K decreased the noise resistance in 4 N and 12 N nitric acid implying higher corrosion rates at higher temperatures. The corrosion rates were similar at 323 K and 348 K in these concentrations. The simultaneous measurement of current and potential noise facilitated the evaluation of the frequency dependence of the noise data to determine the spectral noise resistance (R_sn) and the DC limit of the spectral noise resistance . The results from R_sn and also indicated higher corrosion rates at higher concentration and temperature. Also R_n and correlated well in 4 N and 12 N nitric acid at 323 K and 348 K while disparity was observed at room temperature in 4 N and 12 N nitric acid.     

Automated processing of electrochemical current noise in the time domain: I. Simulated signal

The concept of automated processing of electrochemical current noise in the time domain is presented. The methodology is based on the identification of “run-ups”, which are representative of every single rising feature (transient) in a signal. Selected transients are then subjected to the analysis in terms of their characteristic parameters. Functionality is first demonstrated on a simulated signal with a predefined sample of pitting nucleation events and then in application to processing current signatures from potentiostatic tests on Alloy C22. The results from the analysis of the simulated signal are compared with those of a frequency-based analysis, and it is demonstrated that analysis in the time domain provides direct quantitative information on electrochemical processes that cannot be obtained from frequency-domain analysis.     

Corrosion of the component phases presents in high copper dental amalgams. Application of electrochemical impedance spectroscopy and electrochemical noise analysis

The corrosion resistance of three of the constituent phases in high copper dental amalgams has been investigated by electrochemical methods in 0.9% NaCl solution. Polarization curves show corrosion potentials most positive for γ₁-Ag₂Hg₃, followed by Ag-Cu, and γ-Ag₃Sn in agreement with the order of corrosion resistance deduced from the corrosion currents. Complex plane impedance plots at the open circuit potential showed distorted semicircles with diffusional components at low frequency for Ag-Hg and Ag-Cu, while for γ-Ag₃Sn a layer of corrosion products is formed, partially or completely covering the surface of the electrode. Impedance and noise spectra have been compared in the frequency domain, and show good agreement.     

Modes of inhibiting electrode processes (corrosion included) and their experimental discrimination. I. Definition,nomenclature and classification of modes of inhibition in electrochemical electrode reactions

As to the modes of inhibiting electrode processes, it belongs to the present tasks of the Working Party “Inhibitors” of the European Federation Corrosion first to elaborate valid formulations in definition, nomenclature and classification and second to present a review on methodology for investigating inhibition experimentally. The author is offering contributions to these themes for discussion. Regarding the international character of the European Federation, these contributions were written in English. In the following part I, inhibition of electrode reactions is defined and classified in

• 1 Interface inhibition
• 2 Electrolyte-layer inhibition
• 3 Membrane-inhibition
• 4 Passivation.

Different modes of covering the interface are leading to interface inhibition: Desactivating, indifferent or reactive coverage. Peculiarities in the interface inhibition of electrolytical metal deposition and dissolution are discussed. Electrolyte-layer inhibition is subdivided in a mechanical and an electrochemical species. Envisaging the origin of the inhibiting substances, primary and secondary inhibitors are distinguished. This subdivision is applied to interface and electrolyte-layer inhibition. Even by their nature, membrane inhibition and passivation are secondary. Finally, the inhibition of two or more electrode reactions running down simultaneously and their further classification are treated. It is called special attention to peculiarities of the most important representative of this group, the corrosion inhibition (including vapour phase inhibition). The review on methodology will be given in part II to be published later.